The first electrical device is a device, the operating voltage and/or current of which is high enough to become dangerous for an operator or to cause some damage in the surrounding equipment in case of an electrical fault. Therefore, the electrical behaviour of the first electrical device has to be monitored in order to be able to react quickly enough in case of a fault. In most situations, the power supply will be switched off as soon as a fault is detected.
In the field of power generation, transmission and distribution, the monitoring of the corresponding electrical devices, such as generators, transformers or transmission lines, is essential to ensure reliability and stability of the power grid. In “Dielectric Spectroscopy in Time and Frequency Domain for HV Power Equipment, Part I: Theoretical Considerations” by Zaengl, W. S.; IEEE Electrical Insulation Magazine, vol. 19, issue 5, pages 5-19, the method of dielectric spectroscopy is introduced as a means to diagnose electric insulation materials used in power engineering. Dielectric spectroscopy may also be called impedance spectroscopy, and it is based on the reaction of a material to an applied electromagnetic field at a certain frequency. In most general terms, the response of the material to the applied AC field is determined by measuring the complex voltage and the complex current over the material simultaneously and by determining the impedance of the material at the specific frequency. The dielectric spectroscopy technique, or the dielectric response measurement technique as it may also be called, is further described in “Straight Dielectric Response Measurements with High Precision” conference proceedings from Nord-IS 2005, paper 27 by J Hedberg and T Bengtsson.
In the European patent application EP 1 890 369 A1, the method of dielectric spectroscopy is used to improve the accuracy of a ground fault detection method applied to a stator winding of a multiple-phase generator. A ground fault in the stator winding, which is an internal fault and which can be caused by physical damage or ageing of the insulation material of the winding, precedes generally all other possible faults of the generator, such as phase-to-phase faults of the stator. Accordingly, it is important to detect the ground fault in the winding to prevent more severe faults in the generator. The stator of the generator is connected and the neutral point is connected to ground through an impedance. For the dielectric spectroscopy, an AC test signal is injected between the neutral point and ground and the response of the stator winding to that AC signal is measured. From the measurements, a fault impedance to ground is determined, where the fault impedance is modelled as a fault resistance in parallel to a winding capacitance.
WO 96/05516 discloses a system for monitoring a dual voltage ungrounded system. The monitoring system is configured to determine a fault impedance by injecting test current into a ground terminal to generate a measurement voltage across the fault impedance. The impedance is calculated by using the measurement voltage and the current. In order to determine the impedances of both phases in a dual voltage system, two test currents, having different frequencies, are injected to generate two measurement voltages. The frequencies of the currents are multiples of the system frequency. The test currents and measured voltages are supplied to a micro controller unit, which is configured to calculate the impedance of the two phases. Each of the measured currents and voltages is supplied to an analogue input channel of the micro controller. Thus, the number of input channels needed depends on the number of measurements. In this case four input channels are needed. However, the number of input channels is limited. For example, in a relay box the number of input channels is limited to 12 or 24. Most of those input channels are used for other purposes. Costs are associated with having more channels
In the existing processing devices, which are used in power engineering, measurements are delivered via input channels as analogue signals to the processing device, which transforms the signals into digital data. The processing device usually fulfils several tasks. These tasks are often the controlling, protection and supervision not only of the one electrical device but also of a whole facility. As a result, one such processing device can be quite costly. Since the processing device receives the measurements as analogue signals, the number of input signals is limited. Accordingly, the more measurements that are desired to be processed, the more analogue inputs are required which results in a costly hardware extension. Apart from that, the time and effort used for wiring and installation are increased.
In today's electrical facilities the aspects of increased performance, reliability and safety on one side and of higher efficiency and reduced costs on the other side have to be balanced.